In reading through this thread I would say the decision on purchasing single level or two level cars would be based on who could manufacture them. If the entire Western LD system is forced to go single level; Amtrak would lose much capacity (and revenue) on the sleeper cars. An example of this would be the Superliner with 7 bedrooms (1 H) and 14 roomettes as opposed to the Viewliners with only three bedrooms (1H) and 12 roomettes. That's a substantial difference.
There are plenty of manufacturers that could manufacture bilevel cars. In fact an order as large as Amtrak will make (likely 400-500) would justify companies without an existing platform developing one. Alstom and Stadler are the most capable at the moment in my opinion. Amtrak orders enough equipment that they seem to drive markets more than rely on them.
Amtrak could make up for the loss of capacity with longer trains and/or running a 2nd section on each LD route.
Many platforms are already too short as it is. Plus bilevel cars offer a massive level of flexibility. Superliners can have a full width areas that don't impede movement between cars (Cafe, larger Restroom/lounge, etc). I see having a wheelchair lift as less of a hassle then re-configuring so many platforms and making car layouts less efficient.